One of those was that my phone was intermittently slow. Switching between apps sometimes ground to a halt for a few seconds, and scrolling in Firefox would stutter on long pages.
So I did some searching, and what I found was this, where someone noted the steps that Google's tech support took them through when they phoned to complain about their phone suddenly being very slow.
Now, these are the steps for a Nexus 5, so I am absolutely not saying they will work for any other phone. And if you try this, your phone implodes, and you are left with shards of glass embedded in your ceiling, then it is absolutely not my responsibility.
But having followed these steps, my phone is now working much more smoothly.
You turn off the phone, then hold the power button and volume down to go into the boot loader.Warning: There are people commenting on there saying "I waited for five minutes, thought it had crashed, and rebooted, bricking my phone." If you are going to try this, then be patient. It will probably take _at least_ ten minutes to run through. If you're going to get bored after anything less than an hour then just don't do it.
Then hit volume down till it says Recovery, and hit the power button to select.
Then you will see the android lying flat with the words "No command." underneath.
This part took a few tries, but you now briefly hold down the power button for about half a second to a second (not too long or the phone will restart I think) and WHILE HOLDING DOWN THE POWER BUTTON, tap the VOLUME UP BUTTON and LET GO OF BOTH.
This will bring you to a menu with several options, like loading from ADB (sorry if I said that wrong), along with an option to Wipe Cache. Use volume buttons to scroll down to Wipe Cache and hit power button to select. It will prompt a couple of status messages as its doing so (takes a while, about 10 minutes), and afterwards it will bring you back to the previous menu with the first option now being to Restart the phone. Select that one, give the phone about 10 minutes to not just boot up to the home screen, but let all the startup actions take place, let your apps ping the servers or whatever else, and then you should be good to go.
(And don't ask me for technical support here. I'm just following instructions written by some random person on Reddit. Frankly, that's the stupidest way to do anything technical _ever_, so any advice I have should not be listened to in any circumstances.)
Original post on Dreamwidth - there are comments there.